Electrolysers use electricity to transform reactant chemicals to desired product chemicals through electrochemical reactions, i.e., reactions that occur at electrodes that are in contact with an electrolyte. Hydrogen is a product chemical of increasing demand for use in chemical processes, and also potentially for use in hydrogen vehicles and equipment powered by hydrogen fuel cell engines or hydrogen internal combustion engines (or hybrid hydrogen vehicles, also partially powered by batteries). Water electrolysers, which produce hydrogen and oxygen from water and electricity, are the most common type of electrolyser used for production of gaseous hydrogen as the main product. The most common types of commercial water electrolysers are alkaline water electrolysers (AWE) and polymer electrolyte membrane (PEM) water electrolysers.
As used herein, the terms “half cell”, “half electrolysis cell” and equivalent variations thereof refer to a structure comprising one electrode and its corresponding half cell chamber that provides space for gas-liquid (or gas) flow out of the half cell. The term “cathode half cell” refers to a half cell containing a cathode, and the term “anode half cell” refers to a half cell containing an anode.
As used herein, the terms “cell”, “electrolysis cell” and equivalent variations thereof refer to a structure comprising a cathode half cell and an anode half cell. A cell also includes a separator membrane (referred to herein after as a “membrane”), typically located between, and in close proximity to, in contact with, or integral with, the cathodes and anodes. The functionality of the membrane is to maintain the hydrogen and oxygen gases produced separate and of high purity, while allowing for ionic conduction of electricity between the anode and cathode. A membrane therefore defines one side of each half cell. The other side of each half cell is defined by an electronically conducting solid plate, typically comprised of metal, carbon, carbon-polymer composite, or combinations thereof, and generally known as a bipolar plate. The functionality of the bipolar plate is to maintain the fluids in adjacent half cell chambers of adjacent cells separate, while conducting current electronically between the adjacent cells. Each half cell chamber also contains an electronically conducting component generally known as a current collector or current carrier, to conduct current across the half cell chamber, between the electrode and the bipolar plate.
As used herein, the terms “cell stack”, “electrolyser stack”, “stack”, or equivalent variations thereof refer to structures used for practical (commercial) water electrolysers comprising multiple cells, in which the cells typically are electrically connected in series (although designs using cells connected in parallel and/or series also are known), with bipolar plates physically separating but providing electrical communication between adjacent cells. Gas-liquid (i.e., hydrogen-liquid and oxygen-liquid) mixtures are collected from individual half-cells in header flow passages (top flow manifolds), which run lengthwise along the stack, above the cells. The header flow passages fluidly communicate with respective gas-liquid discharge passages extending through the electrolyser stack and in fluid communication with external piping or tubing, which in turn fluidly communicate with external gas-liquid separation vessels. Operations performed in the external gas-liquid separation vessels include gas-liquid separation, and optionally feed water addition and liquid mixing. Degassed liquid is returned to the cell stack via external piping or tubing, which is in fluid communication with respective degassed liquid return passages extending through the electrolyser stack. Degassed liquid is distributed to individual half-cells via footer flow passages (bottom flow manifolds), which run lengthwise along the stack, underneath the cells. In some PEM electrolyser stacks, the hydrogen side is operated without circulating liquid, in which case the hydrogen side header flow passage(s) and discharge passage(s) would carry hydrogen gas, and in which case there would be no requirement for a gas-liquid separation circuit on the hydrogen side.
As used herein, the term “electrolyser module” refers to the combination of an electrolyser stack and gas-liquid separation spaces in the same structure, which typically is of the filter press type. Further, the term “electrolyser module” as used herein may refer to an alkaline electrolyser module or a PEM electrolyser module. We previously disclosed designs for an alkaline electrolyser module in U.S. Pat. No. 8,308,917, and for a PEM electrolyser module in US 2011/0042228, both of which are incorporated herein by reference.
As used herein, the term “structural plate” refers to a body having a sidewall extending between opposite end faces with a half cell chamber opening, and in the case of an electrolyser module, additionally at least one degassing chamber opening extending through the structural plate between the opposite end faces. An electrolyser stack or an electrolyser module typically is constructed using a series of structural plates to define alternately cathode and anode half cell chambers, fluid flow passages, and in the case of an electrolyser module, at least one degassing chamber, and respective gas-liquid flow passages and respective degassed liquid flow passages extending between the one or more degassing chambers and the corresponding half cell chambers. The structural plates are arranged in face to face juxtaposition between opposite end pressure plates, optionally with at least one intermediate pressure plate interspersed between the structural plates along a length of the electrolyser stack or electrolyser module, to form a filter press type structure. The end pressure plates and intermediate pressure plates can be made of, e.g., one or more of steel, stainless steel, nickel-plated steel, nickel-plated stainless steel, nickel and nickel alloy. The structural plates also hold functional components, which may include, for example, cathodes, anodes, separator membranes, current collectors, and bipolar plates, in their appropriate spatial positions and arrangement.
The structural plates are made of a suitable electrically insulating plastic or fiber-reinforced plastic that is inert to electrolyte (e.g., in the case of an alkaline electrolyser module, an aqueous solution of 25% to 35% KOH at elevated temperatures) or water (in the case of a PEM electrolyser module) and gases (e.g., oxygen, hydrogen, nitrogen). Examples of suitable plastics include polyoxymethylene (POM), polypropylene, polyphenylene oxide (PPO), polyphenylene sulphide (PPS) and the like, and in particular, polysulfone. The structural plates are manufactured by processes such as machining, and more preferably, injection molding, sometimes with some post-machining. Thus, the plates are lightweight, non-conducting, resistant to the operating environment, and amenable to simple and relatively low cost fabrication.
Generally contemplated operating pressures of electrolyser modules and electrolyser stacks lie between atmospheric pressure and 30 barg, and more typically up to 10 barg, depending on the application requirements. Higher pressure operation, for example, in the range of 17 to 30 barg, is advantageous as it enables direct filling of commonly-used gas storage vessels, or a reduced number of mechanical compression stages when filling higher pressure storage. Older electrolyser stack designs utilized steel structural plates, which enabled operation at elevated pressures, e.g., 30 barg, but presented other challenges, such as very high weight, the need for electrical insulation, and potential for corrosion. For modern, “advanced” electrolyser stack and electrolyser module designs utilizing structural plates made of polymeric materials, higher pressure operation presents challenges with regard to mechanical integrity of the structural plates, especially over the long term and for large scale electrolyser modules and electrolyser stacks. External pressure containment means, such as a pressure vessel or a load bearing reinforcing support surrounding an electrolyser stack are known in the art (e.g., U.S. Pat. No. 6,153,083, U.S. Pat. No. 7,314,539), but preferably are to be avoided in order to maintain inherent design simplicity, ease of implementation, compactness, lightweight, and low capital cost. The structural plates could be made significantly more massive, but this approach is impractical and also preferably to be avoided, due to correspondingly significantly increased cost, size, weight, and difficulty of injection molding. The approach of reinforcing each structural plate may be preferred if it can be implemented simply, without significantly adverse effects on ease of assembly, compactness, weight and cost.
U.S. Pat. No. 7,332,063 discloses an approach to reinforcement of individual structural plates in an electrolyser stack in which each structural plate is supported externally by a surrounding external wound fiberglass reinforcement, in order to withstand higher operating pressures. The approach of imposing a tight-fitting external support around the external periphery of structural plates is best suited to circular shapes, such as that contemplated in U.S. Pat. No. 7,332,063. However, for large structural plates with complex irregular shapes, this type of external support would be less effective and more difficult and expensive to install.
Thus, what is needed is a simple, easily-implemented, cost effective approach to reinforcement of individual structural plates for electrolyser modules and electrolyser stacks, especially large-scale electrolyser modules and electrolyser stacks, in order to enable them to operate at higher pressures.